Thanks for your bug report. I have not been able to reproduce this issue. Here are the steps I am taking to test this:

In status window #1:
1) Connect to Efnet
2) Join #channel
3) Set +k password on #channel

In status window #2
1) Connect to Efnet
2) Join #channel
3) mIRC uses the correct password to join #channel

If you follow the above steps exactly, can you reproduce the issue?